1. Field of the Invention
The present invention relates to a double-talk detecting apparatus in which a state of a double talk performed by both a near-end caller and a far-end caller through a transmission path in a two-way communication system is detected. Also, the present invention relates to an echo canceller and an echo suppressor in which an echo occurring on a near-end caller""s side in the double-talk detecting apparatus is cancelled or suppressed.
2. Description of Related Art
A conventional double-talk detecting apparatus is, for example, disclosed in the Japanese Published Unexamined Patent Application No. H10-242891. In this application, an echo canceller, in which a first conventional double-talk detecting method A and a second conventional double-talk detecting method B are performed for the purpose of correctly performing the renewal of filter coefficients used in an adaptive filter, are described.
In detail, in a two-way communication system, a transmission signal is received from a near-end caller, a reception signal is received from a far-end caller, a convolution calculation using the filter coefficients is performed for the reception signal in the adaptive filter to produce a pseudo-echo signal, the pseudo-echo signal is subtracted from the transmission signal in a subtracting unit to produce a residual signal (or called an error signal), and the filter coefficients are renewed according to the residual signal. In the first conventional double-talk detecting method A, the judgment whether or not a double-talk is performed by both the near-end caller and the far-end caller is performed by using three equations (1) to (3) using a root-mean-square level (or an average amplitude level, and called a level hereinafter) S of the transmission signal, a level X of the reception signal and a level E of the residual signal.
X less than p1xe2x80x83xe2x80x83(1)
S greater than p2*X (here, p2xe2x89xa60.5)xe2x80x83xe2x80x83(2)
E greater than p3*Sxe2x80x83xe2x80x83(3)
Here, each of the coefficients P1, p2 and p3 is a constant. Values of the constants P1, p2 and p3 are determined according to environmental conditions of the double-talk detecting apparatus.
The judgment of the double-talk is performed according to three conditions.
First Condition: the equation (1) is satisfied.
Second Condition: the equation (1) is not satisfied, but the equation (2) is satisfied.
Third Condition: neither the equation (1) nor the equation (2) is satisfied, but the equation (3) is satisfied.
In cases where the first condition is satisfied, a voiceless state of the reception signal is detected, so that a single-talk state is judged. In cases where the second or third condition is satisfied, it is judged that the near-end and far-end callers are in a double-talk state.
In the second conventional double-talk detecting method B, the judgment whether or not a double-talk is performed is performed by using two equations (4) to (5).
Sj/Ej less than C2*SM/EM (C2=xc2xd)xe2x80x83xe2x80x83(4)
Sj/Ej less than C1*SM/EM (C1=xc2xc)xe2x80x83xe2x80x83(5)
Here, the symbol SM denotes a level of the transmission signal obtained when the filter coefficients of the adaptive filter are sufficiently converged in a single-talk state in which only the voice of the far-end caller is present (that is, the near-end caller is not speaking), the symbol EM denotes a level of the residual signal at the determination time of the level SM, the symbol Sj denotes a current level of the transmission signal, and the symbol Ej denotes a current level of the residual signal. In cases where the equation (4) or the equation (5) is satisfied, it is judged that the near-end and far-end callers are in a double-talk state at the current time.
Also, in the Application No. H10-242891, a first conventional echo canceller and a second and conventional echo canceller are disclosed. In the first conventional echo canceller, the renewal operation for the filter coefficients of the adaptive filter is stopped in cases where the first condition, the second condition or the third condition is satisfied according to the first conventional double-talk detecting method A. Also, in the second conventional echo canceller, the renewal operation for the filter coefficients of the adaptive filter is stopped in cases where the equation (4) or the equation (5) is satisfied according to the second conventional double-talk detecting method B.
Also, a conventional echo suppressor is, for example, disclosed in the International Telecommunication Union (ITU)-T Recommendation G.164. This echo suppressor is composed of a transmission path loss unit, a receiving line loss unit and a double-talk detecting unit. A line loss in the transmission path loss unit and a line loss in the receiving line loss unit are determined according to a detecting result obtained in the double-talk detecting unit.
Also, another conventional double-talk detecting method is disclosed in FIG. 6/G.164 and FIG. 10/G.164 of the ITU-T Recommendation G.164. In this method, a double-talk is detected by using only two inputs of a reception signal and a transmission signal. Therefore, the accuracy of the detection in this method is inferior to those of the first conventional double-talk detecting method A and the second conventional double-talk detecting method B.
There are two types of state transitions for which the detection of a double-talk state is required.
In a first state transition, a single-talk state, in which only the voice of the far-end caller is present, is changed to a double-talk state in which a voice of the near-end caller is added to the voice of the far-end caller.
In a second state transition, a single-talk state, in which a voice of the near-end caller only exists, is changed to a double-talk state in which a voice of the far-end caller is added to the voice of the near-end caller.
A double-talk state in the case of the second state transition can be detected according to any conventional double-talk detecting method. The reason is that a double-talk state can be detected by judging that a voiceless state of the reception signal is changed to a voiced state when a single-talk state based on the existence of a near-end caller""s voice is changed to a double-talk state. For example, the double-talk state is detected in the first conventional double-talk detecting method A when the second condition is satisfied.
However, the detection of the double-talk state in the case of the first state transition is difficult as compared with that in the case of the second state transition. The reason is that it is required to detect the existence of a near-end caller""s voice, on which an echo signal derived from a reception signal is superposed in a transmission signal, when a single-talk state based on the existence of a far-end caller""s voice is changed to a double-talk state.
Various drawbacks in the detection of the double-talk state in the case of the first state transition are described hereinafter.
Drawbacks in the first conventional double-talk detecting method A are as follows.
(First Drawback) It is required to determine values of the coefficients P2 and P3 used in the equations (2) and (3) in advance according to the environmental conditions. Therefore, the use of the first conventional double-talk detecting method A is limited to a two-way communication system matching with the environmental conditions corresponding to the constants P2 and P3.
(Second Drawback) In cases where a double-talk state is detected because the third condition is satisfied, a ratio of a residual signal level E to a transmission signal level S becomes a value higher than the constant value P3. In this condition of the echo canceller disclosed in the Application No. H10-242891, the filter coefficients of the adaptive filter considerably deteriorate as compared with ideal filter coefficients used to perform an impulse response which is almost the same as an actual impulse response of an echo path through which an echo signal of the reception signal is superposed on a near-end caller""s voice. The pseudo-echo signal which are derived from the reception signal according to a convolution calculation of an impulse response using the deteriorating filter coefficients have an excessive factor corresponding to the deterioration of the filter coefficients as compared with an ideal pseudo-echo signal corresponding to the ideal filter coefficients. As a result, a ratio of the residual signal level E, which is obtained after the subtraction of the subtracting circuit, to the level S of the transmission signal, which is input to the subtracting circuit as an adding factor, becomes a value higher than the constant value P3. Therefore, in this double-talk detecting method A, because the double-talk state is detected when the filter coefficients have considerably deteriorated after a considerable passage of time from the occurrence of the double-talk state, the detection of the double-talk state is necessarily delayed. Therefore, this long detection delay causes various troubles in the first conventional double-talk detecting method A.
Also, a drawback in the second conventional double-talk detecting method B is as follows.
(First Drawback) The levels SM and EM denote average levels of the transmission signal and the residual signal obtained after the filter coefficients are sufficiently converged. Therefore, it is required to judge whether or not the filter coefficients are sufficiently converged. Also, the judgment mainly depends on the detection of the double-talk state. Accordingly, there is a high possibility that the double-talk state is erroneously detected in cases where the judgment of the sufficient conversion for the filter coefficients is performed to detect the double-talk state.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional double-talk detecting apparatus, a double-talk detecting apparatus in which a double-talk state is detected while lessening a detection delay and without depending on any environmental conditions on condition that it is not required to judge whether or not filter coefficients are sufficiently converged. Also, a subordinate object of the present invention is to provide an echo canceller, using the double-talk detecting apparatus, in which an echo of a far-end caller""s voice is cancelled in order to transmit a near-end caller""s voice at a high speech quality while suppressing the deterioration of filter coefficients of an adaptive filter to a minimum. Also, another subordinate object of the present invention is to provide an echo suppressor, using the double-talk detecting apparatus, in which an echo of a far-end caller""s voice is suppressed in a double-talk state while reducing the interruption (or chopping) of a conversation between the far-end caller and a near-end caller.
The object of the present invention is achieved by the provision of a double-talk detecting apparatus, comprising:
first adaptive filtering means for renewing first filter coefficients according to both a first residual signal, which is obtained from a transmission signal, indicates an echo component derived from a reception signal in the single-talk state and indicates the superposition of the echo component and a voice of a first caller in the double-talk state, and a first pseudo-echo signal, which approximates the echo component, and producing the first pseudo-echo signal from the reception signal according to the first filter coefficients renewed, the first filter coefficients deteriorating in the double-talk state;
non-adaptive filtering means for producing a second pseudo-echo signal, in which a deterioration factor indicating the deterioration of the first filter coefficients renewed is included in the double-talk state, from the reception signal according to the first filter coefficients which are renewed by the first adaptive filtering means according to the first residual signal;
delaying means for delaying, by a prescribed time-period, the reception signal which corresponds to a past time prior to a current time by the prescribed time-period and is input to the first adaptive filtering means, the reception signal which corresponds to the past time and is input to the non-adaptive filtering means or the transmission of the first filter coefficients renewed by the first adaptive filtering means at the past time to the non-adaptive filtering means to make the first adaptive filtering means produce the first pseudo-echo signal corresponding to the past time, to make the non-adaptive filtering means produce the second pseudo-echo signal corresponding to the past time or to produce the first filter coefficients corresponding to the past time;
first subtracting means for subtracting the first pseudo-echo signal produced by the first adaptive filtering means from the transmission signal corresponding to the current or past time, which is the same as that of the first pseudo-echo signal, to produce the first residual signal, which does not have the echo component included in the transmission signal in the single-talk state, and transmitting the first residual signal to the first adaptive filtering means;
second subtracting means for subtracting the second pseudo-echo signal produced by the non-adaptive filtering means from the transmission signal corresponding to the current or past time, which is the same as that of the second pseudo-echo signal, to produce a second residual signal which indicates the deterioration factor in the double-talk state and approximates the first residual signal in the single-talk state; and
detecting means for detecting the double-talk state according to a difference between the second residual signal produced by the second subtracting means and the first residual signal produced by the first subtracting means.
In the above configuration, a first pseudo-echo signal is produced from a reception signal by the first adaptive filtering means, and a second pseudo-echo signal is produced from a reception signal by the non-adaptive filtering means according to the renewed first filter coefficients transmitted from the first adaptive filtering means. Thereafter, a first residual signal is produced from the first pseudo-echo signal and a transmission signal by the first subtracting means, a second residual signal is produced from the second pseudo-echo signal and a transmission signal by the second subtracting means, and the double-talk state is detected by the detecting means according to the first residual signal and the second residual signal. In this case, the first pseudo-echo signal or the second pseudo-echo signal is produced from the reception signal corresponding to the past time, or the second pseudo-echo signal is produced according to the renewed first filter coefficients corresponding to the past time. Therefore, a deterioration factor indicating the deterioration of the first filter coefficients renewed is included in the second pseudo-echo signal in the double-talk state.
Accordingly, the transition from the single-talk state to the double-talk state can be detected while lessening a detection delay and without depending on any environmental conditions on condition that it is not required to judge whether or not the first filter coefficients are sufficiently converged.
It is preferred that the delaying means comprises:
first delaying means for receiving the reception signal corresponding to the past time and delaying the reception signal by the prescribed time-period to produce a delayed reception signal, the first pseudo-echo signal being produced from the delayed reception signal by the first adaptive filtering means; and
second delaying means for receiving the transmission signal corresponding to the past time from the first caller and delaying the transmission signal by the prescribed time-period to produce a delayed transmission signal, the first pseudo-echo signal being subtracted from the delayed transmission signal by the first subtracting means,
the second pseudo-echo signal is produced from the reception signal corresponding to the current time by the non-adaptive filtering means,
the second pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the second subtracting means,
the first filter coefficients renewed by the first adaptive filtering means at the current time are transmitted to the non-adaptive filtering means, and
the double-talk detecting apparatus further comprises
third delaying means for delaying the second residual signal produced by the second subtracting means by the prescribed time-period to produce a delayed second residual signal, the double-talk state being detected according to the difference between the delayed second residual signal and the first residual signal.
In the above configuration, in cases where a talk state is in the single-talk state, because no voice of the first caller (for example, a near-end caller) exists in the transmission signal, the first filter coefficients are correctly renewed by the first adaptive filtering means to make the first pseudo-echo signal approximate the echo component, so that the first residual signal is minimized. Also, because the first filter coefficients are correctly renewed, the second pseudo-echo signal produced by the non-adaptive filtering means according to the first filter coefficients renewed approximates the echo component. Therefore, the delayed second residual signal produced from the second pseudo-echo signal becomes almost the same as the first residual signal. Therefore, the difference between the delayed second residual signal and the first residual signal is small. Therefore, no double-talk signal is detected by the detecting means.
In contrast, in cases where the talk state is changed from the single-talk state to the double-talk state, because a voice of the first caller exists with the echo component in the transmission signal, the first filter coefficients are not correctly renewed by the first adaptive filtering means, so that the first filter coefficients renewed deteriorate. In this case, though the first filter coefficients are renewed to reduce the first residual signal regardless of whether or not the first pseudo-echo signal produced by the first adaptive filtering means approximates the echo component, a deterioration factor is included in the second pseudo-echo signal produced by the non-adaptive filtering means because of a time difference between the first residual signal used to renew the first filter coefficients and the reception signal processed by the non-adaptive filtering means. Therefore, the deterioration factor is indicated by the delayed second residual signal, so that the difference between the delayed second residual signal and the first residual signal becomes large. As a result, the double-talk signal is immediately detected by the detecting means according to the difference.
Accordingly, because a deterioration degree of the first filter coefficients is estimated according to a difference between the delayed second residual signal and the first residual signal, it is not required to judge whether or not the first filter coefficients are sufficiently converged, and the double-talk state can be reliably detected while lessening a detection delay and without depending on any environmental conditions.
It is also preferred that the delaying means comprises:
first delaying means for receiving the reception signal corresponding to the past time and delaying the reception signal by the prescribed time-period to produce a delayed reception signal corresponding to the past time, the second pseudo-echo signal being produced from the delayed reception signal by the non-adaptive filtering means; and
second delaying means for receiving the transmission signal corresponding to the past time from the first caller and delaying the transmission signal by the prescribed time-period to produce a delayed transmission signal, the second pseudo-echo signal being subtracted from the delayed transmission signal by the second subtracting means,
the first pseudo-echo signal is produced from the reception signal corresponding to the current time by the first adaptive filtering means,
the first pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the first subtracting means,
the first filter coefficients renewed by the first adaptive filtering means at the current time are transmitted to the non-adaptive filtering means, and
the double-talk detecting apparatus further comprises
third delaying means for delaying the first residual signal produced by the first subtracting means by the prescribed time-period to produce a delayed first residual signal, the double-talk state being detected according to the difference between the second residual signal and the delayed first residual signal.
In the above configuration, because the first filter coefficients renewed by the first adaptive filtering means deteriorate in the double-talk state and because a time difference between the first residual signal used to renew the first filter coefficients and the delayed reception signal processed by the non-adaptive filtering means occurs, a deterioration factor is included in the second pseudo-echo signal, so that a difference between the second residual signal and the delayed first residual signal becomes large in the double-talk state. Therefore, the double-talk state can be immediately detected by the detecting means.
Accordingly, because a deterioration degree of the first filter coefficients is estimated according to a difference between the second residual signal and the delayed first residual signal, it is not required to judge whether or not the first filter coefficients are sufficiently converged, and the double-talk state can be reliably detected while lessening a detection delay and without depending on any environmental conditions.
It is also preferred that the delaying means comprises:
filter coefficient storing means for storing a group of the first filter coefficients renewed by the first adaptive filtering means each time the group of the first filter coefficients is renewed by the first adaptive filtering means and outputting the group of the first filter coefficients corresponding to the past time to the non-adaptive filtering means, the second pseudo-echo signal being produced from the reception signal corresponding to the current time according to the first filter coefficients corresponding to the past time,
the first pseudo-echo signal is produced from the reception signal corresponding to the current time by the first adaptive filtering means,
the first pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the first subtracting means, and
the second pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the second subtracting means.
In the above configuration, the transmission of the group of the first filter coefficients renewed by the first adaptive filtering means to the non-adaptive filtering means is substantially delayed by the filter coefficient storing means. Therefore, the second pseudo-echo signal is produced from the reception signal corresponding to the current time according to the group of the first filter coefficients corresponding to the past time, so that the transition from the single-talk state to the double-talk state is detected.
Accordingly, because a deterioration degree of the first filter coefficients is estimated according to a difference between the second residual signal and the first residual signal, it is not required to judge whether or not the first filter coefficients are sufficiently converged, and the double-talk state can be reliably detected while lessening a detection delay and without depending on any environmental conditions.
It is also preferred that the double-talk detecting apparatus further comprises:
filter coefficient transmission control means for controlling the transmission of a group of the first filter coefficients renewed by the first adaptive filtering means to the filter coefficient storing means according to a double-talk state detecting result obtained by the detecting means each time the group of the first filter coefficients is renewed by the first adaptive filtering means, the group of the first filter coefficients being stored in the filter coefficient storing means in cases where the group of the first filter coefficients is adopted by the filter coefficient transmission control means.
In the above configuration, a group of the first filter coefficients renewed by the first adaptive filtering means is, for example, suddenly changed though the single-talk state is continued. In this case, because it is assumed that the group of the first filter coefficients renewed considerably deteriorates, it is better that the group of the first filter coefficients renewed is abandoned. Also, the steady condition of the reception signal continues, for example, for a long time in the single-talk state, so that a plurality of groups of the first filter coefficients renewed by the first adaptive filtering means are almost the same as each other. In this case, it is better that the groups of the renewed first filter coefficients almost the same as each other are abandoned.
In this double-talk detecting apparatus, it is detected according to the double-talk detecting result whether or not a talk state is the single-talk state or the double-talk state. In cases where it is desired to abandon the group of the currently-renewed first filter coefficients, the group of the currently-renewed first filter coefficients is not adopted.
In contrast, in cases where the group of the currently-renewed first filter coefficients is adopted, the group of the currently-renewed first filter coefficients is stored in the filter coefficient storing means.
Accordingly, because the group of the first filter coefficients currently renewed by the first adaptive filtering means is not adopted in cases where the group of the currently-renewed first filter coefficients is not desired, a memory capacity of the filter coefficient storing means can be reduced, and the double-talk state can be stably detected.
It is also preferred that the non-adaptive filtering means comprises
N non-adaptive filtering units (N is an integer) for respectively producing a j-th pseudo-echo signal (j=2,3, . . . , N+1), in which a deterioration factor indicating the deterioration of the first filter coefficients renewed is included in the double-talk state, from the reception signal corresponding to the current time according to one group of the first filter coefficients stored in the filter coefficient storing means, the N groups of the first filter coefficients being different from each other,
the second subtracting means comprises
N second subtracting units, connected with the N non-adaptive filtering units in one-to-one correspondence, for respectively subtracting the j-th pseudo-echo signal produced by the corresponding non-adaptive filtering unit from the transmission signal corresponding to the current time to produce a j-th residual signal which indicates the deterioration factor in the double-talk state and approximates the first residual signal in the single-talk state, the double-talk state being detected by the detecting means according to a difference between the first residual signal and a group of one or more residual signals selected from the N residual signals.
In the above configuration, N residual signals are produced in the N second subtracting units according to the N groups of first filter coefficients different from each other, and the double-talk state is detected by the detecting means according to a difference between the first residual signal and a group of one or more residual signals selected from the N residual signals.
Accordingly, the transition to the double-talk state can be moreover reliably detected.
Also, because the number of residual signals used for the detection of the double-talk state can be adjusted according to environmental conditions, the double-talk detecting apparatus can be useful for any environmental conditions.
The object of the present invention is achieved by the provision of an echo canceller, comprising:
first adaptive filtering means for renewing first filter coefficients according to both a first residual signal, which is obtained from a transmission signal, indicates an echo component derived from a reception signal in a single-talk state and indicates the superposition of the echo component and a voice of a first caller in a double-talk state, and a first pseudo-echo signal, which approximates the echo component, and producing the first pseudo-echo signal from the reception signal according to the first filter coefficients renewed, the first filter coefficients deteriorating in the double-talk state;
non-adaptive filtering means for producing a second pseudo-echo signal, in which a deterioration factor indicating the deterioration of the first filter coefficients renewed is included in the double-talk state, from the reception signal according to the first filter coefficients which are renewed by the first adaptive filtering means according to the first residual signal;
delaying means for delaying, by a prescribed time-period, the reception signal which corresponds to a past time prior to a current time by the prescribed time-period and is input to the first adaptive filtering means, the reception signal which corresponds to the past time and is input to the non-adaptive filtering means or the transmission of the first filter coefficients renewed by the first adaptive filtering means at the past time to the non-adaptive filtering means to make the first adaptive filtering means produce the first pseudo-echo signal, corresponding to the past time, to make the non-adaptive filtering means produce the second pseudo-echo signal corresponding to the past time or to produce the first filter coefficients corresponding to the past time;
first subtracting means for subtracting the first pseudo-echo signal produced by the first adaptive filtering means from the transmission signal corresponding to the current or past time, which is the same as that of the first pseudo-echo signal, to produce the first residual signal which does not have the echo component included in the transmission signal in the single-talk state and transmitting the first residual signal to the first adaptive filtering means;
second subtracting means for subtracting the second pseudo-echo signal produced by the non-adaptive filtering means from the transmission signal corresponding to the current or past time, which is the same as that of the second pseudo-echo signal, to produce a second residual signal which indicates the deterioration factor in the double-talk state and approximates the first residual signal in the single-talk state;
detecting means for detecting the double-talk state or the single-talk state according to a difference between the second residual signal produced by the second subtracting means and the first residual signal produced by the first subtracting means and outputting a talk state detecting signal indicating the detection of the double-talk state or the detection of the single-talk state;
second adaptive filtering means for renewing second filter coefficients according to, an echo cancelled transmission signal obtained from the transmission signal of the current time and a third pseudo-echo signal approximating the echo component, in cases where the talk state detecting signal indicating the detection of the single-talk state is received from the detecting means, and producing the third pseudo-echo signal corresponding to the current time from the reception signal of the current time according to the second filter coefficients; and
third subtracting means for subtracting the third pseudo-echo signal produced by the second adaptive filtering means from the transmission signal of the current time to produce the echo cancelled transmission signal, in which the echo component included in the transmission signal is removed, sending the echo cancelled transmission signal to the second adaptive filtering means to make the second adaptive filtering means renew the second filter coefficients and outputting the echo cancelled transmission signal.
In the above configuration of the echo canceller, in cases where the detecting means informs the second adaptive filtering means of the single-talk state, the second filter coefficients are renewed, and the echo cancelled transmission signal are output. In contrast, in cases where the detecting means informs the second adaptive filtering means of the double-talk state, no second filter coefficients are renewed, but the echo cancelled transmission signal produced by using the second filter coefficients renewed in the single-talk state are output. Assuming that the second filter coefficients are renewed in the double-talk state, the second filter coefficients deteriorate in the same manner as the first filter coefficients. Therefore, there is no probability that the second filter coefficients deteriorates in the renewal operation in the double-talk state.
Accordingly, the deterioration of the second filter coefficients can be suppressed to a minimum, and the voice of the first caller can be transmitted at a high speech quality even in the double-talk state.
Also, because a deterioration degree of the first filter coefficients is estimated according to a difference between the second residual signal and the first residual signal, it is not required to judge whether or not the first filter coefficients are sufficiently converged, and the double-talk state can be reliably detected while lessening a detection delay and without depending on any environmental conditions.
It is preferred that the delaying means comprises:
first delaying means for receiving the reception signal corresponding to the past time and delaying the reception signal by the prescribed time-period to produce a delayed reception signal, the first pseudo-echo signal being produced from the delayed reception signal by the first adaptive filtering means; and
second delaying means for receiving the transmission signal corresponds to the past time from the first caller and delaying the transmission signal by the prescribed time-period to produce a delayed transmission signal, the first pseudo-echo signal being subtracted from the delayed transmission signal by the first subtracting means,
the second pseudo-echo signal is produced from the reception signal corresponding to the current time by the non-adaptive filtering means,
the second pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the second subtracting means,
the first filter coefficients renewed by the first adaptive filtering means at the current time are transmitted to the non-adaptive filtering means, and
the echo canceller further comprises
third delaying means for delaying the second residual signal produced by the second subtracting means by the prescribed time-period to produce a delayed second residual signal, the double-talk state being detected according to the difference between the delayed second residual signal and the first residual signal.
In the above configuration, the first residual signal is produced from the transmission signal corresponding to the past time and the reception signal corresponding to the past time, the first filter coefficients are renewed according to the first residual signal corresponding to the past time, and the second pseudo-echo signal is produced from the reception signal corresponding to the current time according to the first filter coefficients corresponding to the past time.
Accordingly, the transition from the single-talk state to the double-talk state can be reliably detected.
It is preferred that the delaying means comprises:
first delaying means for receiving the reception signal corresponding to the past time and delaying the reception signal by the prescribed time-period to produce a delayed reception signal corresponding to the past time, the second pseudo-echo signal being produced from the delayed reception signal by the non-adaptive filtering means; and
second delaying means for receiving the transmission signal corresponds to the past time from the first caller and delaying the transmission signal by the prescribed time-period to produce a delayed transmission signal, the second- pseudo-echo signal being subtracted from the delayed transmission signal by the second subtracting means, the first pseudo-echo signal is produced from the reception signal corresponding to the current time by the first adaptive filtering means,
the first pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the first subtracting means,
the first filter coefficients renewed by the first adaptive filtering means at the current time are transmitted to the non-adaptive filtering means, and
the echo canceller further comprises
third delaying means for delaying the first residual signal produced by the first subtracting means by the prescribed time-period to produce a delayed first residual. signal, the double-talk state being detected according to the difference between the second residual signal and the delayed first residual signal.
In the above configuration, the first residual signal is produced from the transmission signal corresponding to the current time and the reception signal corresponding to the current time, the first filter coefficients are renewed according to the first residual signal corresponding to the current time, and the second pseudo-echo signal is produced from the reception signal corresponding to the past time according to the first filter coefficients corresponding to the current time.
Accordingly, the transition from the single-talk state to the double-talk state can be reliably detected.
It is preferred that the delaying means comprises:
filter coefficient storing means for storing a group of the first filter coefficients renewed by the first adaptive filtering means each time the group of the first filter coefficients is renewed by the first adaptive filtering means and outputting the group of the first filter coefficients corresponding to the past time to the non-adaptive filtering means, the second pseudo-echo signal being produced from the reception signal corresponding to the current time according to the first filter coefficients corresponding to the past time,
the first pseudo-echo signal is produced from the reception signal corresponding to the current time by the first adaptive filtering means,
the first pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the first subtracting means, and
the second pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the second subtracting means.
In the above configuration, the transmission of the group of the first filter coefficients renewed by the first adaptive filtering means to the non-adaptive filtering means is substantially delayed by the filter coefficient storing means. Therefore, the second pseudo-echo signal is produced from the reception signal corresponding to the current time according to the group of the first filter coefficients corresponding to the past time.
Accordingly, the transition from the single-talk state to the double-talk state can be reliably detected.
It is preferred that the echo canceller further comprises:
filter coefficient transmission control means for controlling the transmission of a group of the first filter coefficients renewed by the first adaptive filtering means to the filter coefficient storing means according to the talk state detecting signal output from the detecting means each time the group of the first filter coefficients is renewed by the first adaptive filtering means, the group of the first filter coefficients being stored in the filter coefficient storing means in cases where the group of the first filter coefficients is adopted by the filter coefficient transmission control means.
In the above configuration, in cases where it is judged according to the talk state detecting signal that it is desired to abandon the group of the currently-renewed first filter coefficients, the group of the currently-renewed first filter coefficients is not adopted. In contrast, in cases where the group of the currently-renewed first filter coefficients is adopted, the group of the currently-renewed first filter coefficients is stored in the filter coefficient storing means.
Accordingly, because the group of the first filter coefficients currently renewed by the first adaptive filtering means is not adopted in cases where the group of the currently-renewed first filter coefficients is not desired, a memory capacity of the filter coefficient storing means can be reduced, and the double-talk state can be stably detected.
It is also preferred that the non-adaptive filtering means comprises
N non-adaptive filtering units (N is an integer) for respectively producing a j-th pseudo-echo signal (j=2,3, . . . , N+1), in which a deterioration factor indicating the deterioration of the first filter coefficients renewed is included in the double-talk state, from the reception signal corresponding to the current time according to one group of the first filter coefficients stored in the filter coefficient storing means, the N groups of the first filter coefficients being different from each other,
the second subtracting means comprises
N second subtracting units, connected with the N non-adaptive filtering units in one-to-one correspondence, for respectively subtracting the j-th pseudo-echo signal produced by the corresponding non-adaptive filtering unit from the transmission signal corresponding to the current time to produce a j-th residual signal which indicates the deterioration factor in the double-talk state and approximates the first residual signal in the single-talk state, the double-talk state being detected by the detecting means according to a difference between the first residual signal and a group of one or more residual signals selected from the N residual signals.
In the above configuration, N residual signals are produced in the N second subtracting units according to the N groups of first filter coefficients different from each other, and the double-talk state is detected by the detecting means according to a difference between the first residual signal and a group of one or more residual signals selected from the N residual signals.
Accordingly, the transition to the double-talk state can be moreover reliably detected.
Also, because the number of residual signals used for the detection of the double-talk state can be adjusted according to environmental conditions, the echo canceller can be useful for any environmental conditions.
The object of the present invention is also achieved by the provision of an echo canceller, comprising:
first adaptive filtering means for renewing first filter coefficients according to an echo cancelled transmission signal, which is obtained from a transmission signal, indicates an echo component derived from a reception signal in a single-talk state and indicates the superposition of the echo component and a voice of a first caller in a double-talk state, and a first pseudo-echo signal, which approximates the echo component, in cases where a talk state detecting signal indicating the detection of the single-talk state is received, and producing the first pseudo-echo signal from the reception signal according to the first filter coefficients renewed, the first filter coefficients deteriorating in the double-talk state;
non-adaptive filtering means for producing a second pseudo-echo signal, in which a deterioration factor indicating the deterioration of the first filter coefficients renewed is included in the double-talk state, from the reception signal according to the first filter coefficients which are renewed by the first adaptive filtering means according to the echo cancelled transmission signal;
delaying means for delaying, by a prescribed time-period, the reception signal which corresponds to a past time prior to a current time by the prescribed time-period and is input to the first adaptive filtering means, the reception signal which corresponds to the past time and is input to the non-adaptive filtering means or the transmission of the first filter coefficients, which are renewed by the first adaptive filtering means at the past time, to the non-adaptive filtering means to make the first adaptive filtering means produce the first pseudo-echo signal corresponding to the past time, to make the non-adaptive filtering means produce the second pseudo-echo signal corresponding to the past time or to produce the first filter coefficients corresponding to the past time;
first subtracting means for subtracting the first pseudo-echo signal produced by the first adaptive filtering means from the transmission signal corresponding to the current or past time, which is the same as that of the first pseudo-echo signal, to produce the echo cancelled transmission signal, which does not have the echo component included in the transmission signal in the single-talk state, and transmitting the echo cancelled transmission signal to the first adaptive filtering means;
second subtracting means for subtracting the second pseudo-echo signal produced by the non-adaptive filtering means from the transmission signal corresponding to the current or past time, which is the same as that of the second pseudo-echo signal, to produce a first residual signal which indicates the deterioration factor in the double-talk state and approximates the echo cancelled transmission signal in the single-talk state; and
detecting means for detecting the double-talk state or the single-talk state according to a difference between the first residual signal produced by the second subtracting means and the echo cancelled transmission signal produced by the first subtracting means and outputting a talk state detecting signal indicating the detection of the double-talk state or the talk state detecting signal indicating the detection of the single-talk state to the first adaptive filtering means.
In the above configuration of the echo canceller, the double-talk state or the single-talk state is detected by the detecting means in the same manner as in the double-talk detecting apparatus, and the talk state detecting signal indicating the detection of the single-talk state is sent to the first adaptive filtering means. In the first adaptive filtering means, no first filter coefficients are renewed in the double-talk, but the first filter coefficients are renewed in the single-talk state according to the talk state detecting signal. Therefore, no first filter coefficients deteriorate in the renewal operation, but the first filter coefficients deteriorate a little bit in the double-talk state because no renewal of the first filter coefficients is performed in the double-talk state. As a result, the double-talk state or the single-talk state can be detected by the detecting means in the same manner as in the double-talk detecting apparatus.
Accordingly, the deterioration of the first filter coefficients can be suppressed to a minimum. Therefore, the voice of the first caller can be transmitted at a high speech quality even in the double-talk state.
Also, because the double-talk state in the first state transition is detected in the same manner as in the double-talk detecting apparatus, it is not required to judge whether or not the first filter coefficients are sufficiently converged, and the double-talk state can be reliably detected while lessening a detection delay and without depending on any environmental conditions.
Also, because neither second adaptive filtering means nor the third subtracting means is not required, the echo canceller, in which a date processing volume and a memory capacity are reduced, can be obtained as compared with the above-described echo cancellers.
It is preferred that the delaying means comprises:
first delaying means for receiving the reception signal corresponding to the past time and delaying the reception signal by the prescribed time-period to produce a delayed reception signal, the first pseudo-echo signal being produced from the delayed reception signal by the first adaptive filtering means; and
second delaying means for receiving the transmission signal corresponds to the past time from the first caller and delaying the transmission signal by the prescribed time-period to produce a delayed transmission signal, the first pseudo-echo signal being subtracted from the delayed transmission signal by the first subtracting means,
the second pseudo-echo signal is produced from the reception signal corresponding to the current time by the non-adaptive filtering means,
the second pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the second subtracting means,
the first filter coefficients renewed by the first adaptive filtering means at the current time are transmitted to the non-adaptive filtering means, and
the echo canceller further comprises
third delaying means for delaying the first residual signal produced by the second subtracting means by the prescribed time-period to produce a delayed first residual signal, the double-talk state being detected according to the difference between the delayed first residual signal and the echo cancelled transmission signal.
In the above configuration, the first residual signal is produced from the transmission signal corresponding to the past time and the reception signal corresponding to the past time, the first filter coefficients are renewed according to the first residual signal corresponding to the past time, and the second pseudo-echo signal is produced from the reception signal corresponding to the current time according to the first filter coefficients corresponding to the past time.
Accordingly, the transition.from the single-talk state to the double-talk state can be reliably detected.
It is preferred that the delaying means comprises:
first delaying means for receiving the reception signal corresponding to the past time and delaying the reception signal by the prescribed time-period to produce a delayed reception signal corresponding to the past time, the second pseudo-echo signal being produced from the delayed reception signal by the non-adaptive filtering means; and
second delaying means for receiving the transmission signal corresponds to the past time from the first caller and delaying the transmission signal by the prescribed time-period to produce a delayed transmission signal, the second pseudo-echo signal being subtracted from the delayed transmission signal by the second subtracting means,
the first pseudo-echo signal is produced from the reception signal corresponding to the current time by the first adaptive filtering means,
the first pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the first subtracting means,
the first filter coefficients renewed by the first adaptive filtering means at the current time are transmitted to the non-adaptive filtering means, and
the echo canceller further comprises
third delaying means for delaying the echo cancelled transmission signal produced by the first subtracting means by the prescribed time-period to produce a delayed second residual signal, the double-talk state being detected according to the difference between the first residual signal and the delayed second residual signal.
In the above configuration, the first residual signal is produced from the transmission signal corresponding to the current time and the reception signal corresponding to the current time, the first filter coefficients are renewed according to the first residual signal corresponding to the current time, and the second pseudo-echo signal is produced from the reception signal corresponding to the past time according to the first filter coefficients corresponding to the current time.
Accordingly, the transition from the single-talk state to the double-talk state can be reliably detected.
It is preferred that the delaying means comprises
filter coefficient storing means for storing a group of the first filter coefficients renewed by the first adaptive filtering means each time the group of the first filter coefficients is renewed by the first adaptive filtering means and outputting the group of the first filter coefficients corresponding to the past time to the non-adaptive filtering means, the second pseudo-echo signal being produced from the reception signal corresponding to the current time according to the first filter coefficients corresponding to the past time,
the first pseudo-echo signal is produced from the reception signal corresponding to the current time by the first adaptive filtering means,
the first pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the first subtracting means, and
the second pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the second subtracting means.
In the above configuration, the transmission of the group of the first filter coefficients renewed by the first adaptive filtering means to the non-adaptive filtering means is substantially delayed by the filter coefficient storing means. Therefore, the second pseudo-echo signal is produced from the reception signal corresponding to the current time according to the group of the first filter coefficients corresponding to the past time.
Accordingly, the transition from the single-talk state to the double-talk state can be reliably detected.
It is also preferred that the echo canceller further comprises:
filter coefficient transmission control means for controlling the transmission of a group of the first filter coefficients renewed by the first adaptive filtering means to the filter coefficient storing means according to the talk state detecting signal output from the detecting means each time the group of the first filter coefficients is renewed by the first adaptive filtering means, the group of the first filter coefficients being stored in the filter coefficient storing means in cases where the group of the first filter coefficients is adopted by the filter coefficient transmission control means.
In the above configuration, in cases where it is judged according to the talk state detecting signal that it is desired to abandon the group of the currently-renewed first filter coefficients, the group of the currently-renewed first filter coefficients is not adopted. In contrast, in cases where the group of the currently-renewed first filter coefficients is adopted, the group of the currently-renewed first filter coefficients is stored in the filter coefficient storing means.
Accordingly, because the group of the first filter coefficients currently renewed by the first adaptive filtering means is not adopted in cases where the group of the currently-renewed first filter coefficients is not desired, a memory capacity of the filter coefficient storing means can be reduced, and the double-talk state can be stably detected.
It is also preferred that the non-adaptive filtering means comprises
N non-adaptive filtering units (N is an integer) for respectively producing a j-th pseudo-echo signal (j=2,3, . . . , N+1), in which a deterioration factor indicating the deterioration of the first filter coefficients renewed is included in the double-talk state, from the reception signal corresponding to the current time according to one group of the first filter coefficients stored in the filter coefficient storing means, the N groups of the first filter coefficients being different from each other,
the second subtracting means comprises
N second subtracting units, connected with the N non-adaptive filtering units in one-to-one correspondence, for respectively subtracting the j-th pseudo-echo signal produced by the corresponding non-adaptive filtering unit from the transmission signal corresponding to the current time to produce a j-th residual signal which indicates the deterioration factor in the double-talk state and approximates the echo cancelled transmission signal in the single-talk state, the double-talk state being detected by the detecting means according to a difference between the echo cancelled transmission signal and a group of one or more residual signals selected from the N residual signals.
In the above configuration, N residual signals are produced in the N second subtracting units according to the N groups of first filter coefficients different from each other, and the double-talk state is detected by the detecting means according to a difference between the first residual signal and a group of one or more residual signals selected from the N residual signals.
Accordingly, the transition to the double-talk state can be moreover reliably detected.
Also, because the number of residual signals used for the detection of the double-talk state can be adjusted according to environmental conditions, the echo canceller can be useful for any environmental conditions.
The object of the present invention is achieved by the provision of an echo suppressor, comprising:
first adaptive filtering means for renewing first filter coefficients according to both a first residual signal, which is obtained from a transmission signal, indicates an echo component derived from a reception signal in a single-talk state and indicates the superposition of the echo component and a voice of a first caller in a double-talk state, and a first pseudo-echo signal, which approximates the echo component, and producing the first pseudo-echo signal from the reception signal according to the first filter coefficients renewed, the first filter coefficients deteriorating in the double-talk state;
non-adaptive filtering means for producing a second pseudo-echo signal, in which a deterioration factor indicating the deterioration of the first filter coefficients renewed is included in the double-talk state, from the reception signal according to the first filter coefficients which are renewed by the first adaptive filtering means according to the first residual signal;
delaying means for delaying, by a prescribed time-period, the reception signal which corresponds to a past time prior to a current time by the prescribed time-period and is input to the first adaptive filtering means, the reception signal which corresponds to the past time and is input to the non-adaptive filtering means or the transmission of the first filter coefficients, which are renewed by the first adaptive filtering means at the past time, to the non-adaptive filtering means to make the first adaptive filtering means produce the first pseudo-echo signal corresponding to the past time, to make the non-adaptive filtering means produce the second pseudo-echo signal corresponding to the past time or to produce the first filter coefficients corresponding to the past time;
first subtracting means for subtracting the first pseudo-echo signal produced by the first adaptive filtering means from the transmission signal corresponding to the current or past time, which is the same as that of the first pseudo-echo signal, to produce the first residual signal, which does not have the echo component included in the transmission signal in the single-talk state, and transmitting the first residual signal to the first adaptive filtering means;
second subtracting means for subtracting the second pseudo-echo signal produced by the non-adaptive filtering means from the transmission signal corresponding to the current or past time, which is the same as that of the second pseudo-echo signal, to produce a second residual signal which indicates the deterioration factor in the double-talk state and approximates the first residual signal in the single-talk state;
detecting means for detecting the double-talk state or the single-talk state according to a difference between the second residual signal produced by the second subtracting means and the first residual signal produced by the first subtracting means and outputting a talk state detecting signal indicating the detection of the double-talk state or the detection of the single-talk state;
attenuation control, means for receiving the talk state detecting signal from the detecting means, generating a transmission attenuation control signal indicating an attenuation degree of the transmission signal currently received according to the talk state detecting signal, and generating a reception attenuation control signal indicating an attenuation degree of the reception signal currently received according to the talk state detecting signal;
first attenuating means for attenuating the transmission signal currently received according to the transmission attenuation control signal generated by the attenuation control means to produce an attenuated transmission signal and outputting the attenuated transmission signal; and
second attenuating means for attenuating the reception signal currently received according to the reception attenuation control signal generated by the attenuation control means to produce an attenuated reception signal and outputting the attenuated reception signal.
In the above configuration of the echo suppressor, when the single-talk state based on a second caller""s voice existing in the reception signal is changed to the double-talk state because a voice of the first caller is added to the transmission signal, the double-talk state is detected by the detecting means, and the reception signal is attenuated according to the reception attenuation control signal. Therefore, an echo component superposed on a voice of the first caller can be suppressed. Also, in this case, an attenuation degree of the transmission signaling the double-talk state is reduced as compared with that in the single-talk state.
Accordingly, because the attenuation degrees of the transmission signal and the reception signal are adjusted according to the attenuation control signals, an echo of a second caller""s voice can be suppressed in the double-talk state while reducing the interruption (or chopping) of a conversation, and the conversation can be performed at a high speech quality.
Also, because the double-talk state in the first state transition is detected in the same manner as in the double-talk detecting apparatus, it is not required to judge whether or not the first filter coefficients are sufficiently converged, and the double-talk state can be reliably detected in the first state transition while lessening a detection delay and without depending on any environmental conditions.
It is preferred that the delaying means comprises:
first delaying means for receiving the reception signal corresponding to the past time and delaying the reception signal by the prescribed time-period to produce a delayed reception signal, the first pseudo-echo signal being produced from the delayed reception signal by the first adaptive filtering means; and
second delaying means for receiving the transmission signal corresponding to the past time from the first caller and delaying the transmission signal by the prescribed time-period to produce a delayed transmission signal, the first pseudo-echo signal being subtracted from the delayed transmission signal by the first subtracting means,
the second pseudo-echo signal is produced from the reception signal corresponding to the current time by the non-adaptive filtering means,
the second pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the second subtracting means,
the first filter coefficients renewed by the first adaptive filtering means at the current time are transmitted to the non-adaptive filtering means, and
the echo suppressor further comprises
third delaying means for delaying the second residual signal produced by the second subtracting means by the prescribed time-period to produce a delayed second residual signal, the double-talk state being detected according to the difference between the delayed second residual signal and the first residual signal.
In the above configuration, the first residual signal is produced from the transmission signal corresponding to the past time and the reception signal corresponding to the past time, the first filter coefficients are renewed according to the first residual signal corresponding to the past time, and the second pseudo-echo signal is produced from the reception signal corresponding to the current time according to the first filter coefficients corresponding to the past time.
Accordingly, the transition from the single-talk state to the double-talk state can be reliably detected.
It is preferred that the delaying means comprises:
first delaying means for receiving the reception signal corresponding to the past time and delaying the reception, signal by the prescribed time-period to produce a delayed reception signal corresponding to the past time, the second pseudo-echo signal being produced from the delayed reception signal by the non-adaptive filtering means; and
second delaying means for receiving the transmission signal corresponds to the past time from the first caller and delaying the transmission signal by the prescribed time-period to produce a delayed transmission signal, the second pseudo-echo signal being subtracted from the delayed transmission signal by the second subtracting means,
the first pseudo-echo signal is produced from the reception signal corresponding to the current time by the first adaptive filtering means,
the first pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the first subtracting means,
the first filter coefficients renewed by the first adaptive filtering means at the current time are transmitted to the non-adaptive filtering means, and
the echo suppressor further comprises
third delaying means for delaying the first residual signal produced by the first subtracting means by the prescribed time-period to produce a delayed first residual signal, the double-talk state being detected according to the difference between the second residual signal and the delayed first residual signal.
In the above configuration, the first residual signal is produced from the transmission signal corresponding to the current time and the reception signal corresponding to the current time, the first filter coefficients are renewed according to the first residual signal corresponding to the current time, and the second pseudo-echo signal is produced from the reception signal corresponding to the past time according to the first filter coefficients corresponding to the current time.
Accordingly, the transition from the single-talk state to the double-talk state can be reliably detected.
It is preferred that the delaying means comprises:
filter coefficient storing means for storing a group of the first filter coefficients renewed by the first adaptive filtering means each time the group of the first filter coefficients is renewed by the first adaptive filtering means and outputting the group of the first filter coefficients corresponding to the past time to the non-adaptive filtering means, the second pseudo-echo signal being produced from the reception signal corresponding to the current time according to the first filter coefficients corresponding to the past time,
the first pseudo-echo signal is produced from the reception signal corresponding to the current time by the first adaptive filtering means,
the first pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the first subtracting means, and
the second pseudo-echo signal is subtracted from the transmission signal corresponding to the current time by the second subtracting means.
In the above configuration, the transmission of the group of the first filter coefficients renewed by the first adaptive filtering means to the non-adaptive filtering means is substantially delayed by the filter coefficient storing means. Therefore, the second pseudo-echo signal is produced from the reception signal corresponding to the current time according to the group of the first filter coefficients corresponding to the past time.
Accordingly, the transition from the single-talk state to the double-talk state can be reliably detected.
It is preferred that the echo suppressor further comprises:
filter coefficient transmission control means for controlling the transmission of a group of the first filter coefficients renewed by the first adaptive filtering means to the filter coefficient storing means according to the talk state detecting signal output from the detecting means each time the group of the first filter coefficients is renewed by the first adaptive filtering means, the group of the first filter coefficients being stored in the filter coefficient storing means in cases where the group of the first filter coefficients is adopted by the filter coefficient transmission control means.
In the above configuration, in cases where it is judged according to the talk state detecting signal that it is desired to abandon the group of the currently-renewed first filter coefficients, the group of the currently-renewed first filter coefficients is not adopted. In contrast, in cases where the group of the currently-renewed first filter coefficients is adopted, the group of the currently-renewed first filter coefficients is stored in the filter coefficient storing means.
Accordingly, because the group of the first filter coefficients currently renewed by the first adaptive filtering means is not adopted in cases where the group of the currently-renewed first filter coefficients is not desired, a memory capacity of the filter coefficient storing means can be reduced, and the double-talk state can be stably detected.
It is preferred that the non-adaptive filtering means comprises
N non-adaptive filtering units (N is an integer) for respectively producing a j-th pseudo-echo signal (j=2,3, . . . , N+1), in which a deterioration factor indicating the deterioration of the first filter coefficients renewed is included in the double-talk state, from the reception signal corresponding to the current time according to one group of the first filter coefficients stored in the filter coefficient storing means, the N groups of the first filter coefficients being different from each other,
the second subtracting means comprises
N second subtracting units, connected with the N non-adaptive filtering units in one-to-one correspondence, for respectively subtracting the j-th pseudo-echo signal produced by the corresponding non-adaptive filtering unit from the transmission signal corresponding to the current time to produce a j-th residual signal which indicates the deterioration factor in the double-talk state and approximates the first residual signal in the single-talk state, the double-talk state being detected by the detecting means according to a difference between the first residual signal and a group of one or more residual signals selected from the N residual signals.
In the above configuration, N residual signals are produced in the N second subtracting units according to the N groups of first filter coefficients different from each other, and the double-talk state is detected by the detecting means according to a difference between the first residual signal and a group of one or more residual signals selected from the N residual signals.
Accordingly, the transition to the double-talk state can be moreover reliably detected.
Also, because the number of residual signals used for the detection of the double-talk state can be adjusted according to environmental conditions, the echo suppressor can be useful for any environmental conditions.